nopackages: Add class for recipes which don't generate packages
It turns out writing the same list of packaging tasks multiple
times in multiple places is error prone. Move this to a new class
'nopackages", migrate existing users and add glibc-initial and
libgcc-initial since we don't want packages for those recipes.
This means the sstate for those recipes won't be installed, saving
small amounts of build time and bandwidth.
A reference to the old package_write task is also dropped.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
